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(meteorobs) (Off Topic) Comets Pummeled Earth 36 Million Years Ago



From the Planetary Society
http://planetary.org/articlearchive/headlines/1998/headln-052198.html

                     Comets Pummeled Earth 36 Million Years Ago

                     Posthumous Paper by Astronomer Gene Shoemaker
                     Details Evidence of Cataclysmic Comet Shower
                                  May 20, 1998

                  Geochemical evidence from a rock quarry in
                  northern Italy indicates that a shower of comets
                  hit Earth about 36 million years ago.

                  The findings not only account for the huge craters
                  at Popagai in Siberia and at Chesapeake Bay in
                  Maryland, but posit that they were but a tiny
                  fraction of the comets active during a period of
                  two or three million years during the late Eocene
                  period. The work provides indirect evidence that a
                  gravitational perturbation of the Oort comet cloud
                  outside the orbit of Pluto was responsible for
                  sending a wave of comets swarming toward the
                  center of the solar system.

                  Shoemaker's Legacy of Discovery

                  In a paper published today in the journal Science,
                  a group from the California Institute of
                  Technology, the United States Geological Survey
                  Flagstaff office, and the Coldigioco Geological
                  Observatory in Italy, report their evidence of a
                  very large increase in the amount of
                  extraterrestrial dust hitting Earth in the late
                  Eocene period. The writers include the
                  husband-and-wife team of Gene and Carolyn
                  Shoemaker, well known for their work detecting
                  comets and asteroids. Gene Shoemaker died in a car
                  crash last year while this research was in
                  progress.

                  According to lead author Ken Farley, a geochemist
                  at Caltech, the contribution of Shoemaker was
                  especially crucial in the breakthrough.

                  "Basically, Gene saw my earlier work and
                  recognized it as a new way to test an important
                  question: Are large impact craters on Earth
                  produced by collisions with comets or asteroids?"
                  Farley says.

                  "He suggested we study a quarry near Massignano,
                  Italy, where sea-floor deposits record debris
                  related to the large impact events 36 million
                  years ago. He said that if there had been a comet
                  shower, the technique I've been working on might
                  show it clearly in these sediments."

                  Carolyn Shoemaker said that she and her husband
                  went to Italy last year to perform field work in
                  support of the paper.

                  Tracking an Ancient Disaster

                  In geologic samples, the researchers detected a
                  helium isotope known as 3He, which is rare on
                  Earth but common in extraterrestrial materials.
                  This isotope is abundant in the Sun, and some of
                  it is ejected from the Sun as solar wind
                  throughout the solar system. The helium is easily
                  picked up and carried along by extraterrestrial
                  objects such as asteroids and comets and their
                  associated dust particles.

                  Thus, arrival of extraterrestrial matter on
                  Earth's surface can be detected by measuring its
                  associated 3He. And even this material is unlikely
                  to include large objects like asteroids and
                  comets. Because these heavy, solid objects fall
                  into the atmosphere with a high velocity, they
                  melt or vaporize, giving their helium up to the
                  atmosphere. This 3He never falls below very high
                  altitudes, and soon reenters space.

                  But tiny particles entering the atmosphere are
                  another story. These particles can pass through
                  the atmosphere at low temperatures, and so retain
                  helium. These particles accumulate on the sea
                  floor, and sea floor sediments provide an archive
                  of these particles going back hundreds of millions
                  of years.

                  Elevated levels of 3He would suggest an unusually
                  dusty inner solar system, possibly because of a
                  flurry of active comets. Such an elevated
                  abundance of comets might arise when a passing
                  star or other gravity anomaly kicks a huge number
                  of comets from the Oort cloud into elliptical,
                  sun-approaching orbits.

                  Discovery in Italy

                  When Farley took Shoemaker's suggestion and
                  traveled to the Italian quarry, he discovered that
                  there was indeed an elevated flux of 3He-laced
                  materials in a sedimentary layer some 50 feet
                  beneath the surface. Because this region of Italy
                  was submerged in water until about 10 million
                  years ago, the comet impacts and microscopic
                  debris had accumulated on the ocean bed, and this
                  debris was preserved because dying organisms had
                  cooperatively covered the debris over the eons.

                  The depth of the sedimentary layer suggested to
                  the researchers that the 3He had been deposited
                  about 36 million years ago. This corresponds to
                  the dating of the craters at Popagai and
                  Chesapeake Bay.

                  More precisely, the 3He measurements show enhanced
                  solar system dustiness associated with the impacts
                  36 million years ago, but with the dustiness
                  beginning 0.5 million years before the impacts and
                  continuing for about 1.5 million years after. The
                  conclusion is that there were a large number of
                  Earth-crossing comets and much dust from their
                  tails for a period of about 2.5 million years.

                  In addition to Gene and Carolyn Shoemaker and Ken
                  Farley, the paper was cowritten by Alessandro
                  Montanari, who holds joint appointments at the
                  Coldigioco Geological Observatory in Apiro, Italy,
                  and the School of Mines in Paris.
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